Due to their higher overall luminous efficacy and flexibility for achieving various light patterns, luminaires using high-flux LEDs are fast emerging as the preferred lighting architecture over conventional light fixtures. These luminaires are increasingly used in a wide range of applications where high light output is required, such as theatrical spotlights, high-power flashlights, and automotive headlights. They are also penetrating mainstream commercial applications like task lights, accent lights, wall washing, signage, advertising, decorative and display lighting, cove lighting, wall sconces, facade lighting, and custom lighting.
The ability to maximize light output from a luminaire increases energy efficiency and reduces production and maintenance costs. Typically, a high flux LED luminaire comprises a plurality of high flux light-emitting diodes, as well as a power supply unit for excitation of the light-emitting diodes. Through maximizing the light output in the desired light pattern, power consumption for these light-emitting diodes may be reduced. Otherwise, additional power would be needed to overcome these light losses.
A primary concern in the design and operation of high flux LED luminaires is thermal management. The luminous intensity of a light module is quite often a strong function of its operational temperature. High flux LED luminaires tend to generate large amounts of heat during operation. Not only does this heat reduce the light output of a light-emitting diode, but it can also reduce the reliability and the life expectancy of the lighting module, due to premature failure of one or more light-emitting diodes. Accordingly, heat dissipation often becomes a critical design consideration as the undesirable heat negatively affects the performance of the luminaire.
Various heat dispersive systems such as heat sinks, use of metal-core printed circuit boards, heat absorbers or a combination thereof have been proposed. However, the existing heat dissipation systems generally spread the heat from a hot spot to another location for dissipation without coolth collection.
For example, U.S. Pat. No. 6,211,626 to Lys et al. discloses a heat dissipating housing made of a heat-conductive material for containing a lighting assembly therein. The heat dissipating housing contains two stacked circuit boards holding respectively a power module and a light module. The light module comprises a light emitting diode (LED) system mounted on a heat spreader plate that is in contact with the housing for dispersing away the heat generated by the LED system that is in thermal contact with the plate, thereby conducting heat towards the housing.
A particular advantage of the Lys et al. heat spreader is that when the heat source is located proximate to the center of a circular plate, the temperature at the boundary thereof is substantially constant. Accordingly, the heat spreader distributes the heat evenly to a thermally connected housing which ejects the heat into the surrounding environment. However, this heat dissipation system may not work well with housings which exhibit hot spots when dissipating heat.
U.S. Pat. No. 4,729,076 to Masami et al. teaches a heat dissipation mechanism for an LED traffic signal. A heat absorber such as a heat conductive resin in thermal communication with a printed circuit board on the other side of which an array of LEDs is formed, is disclosed. A finned heat sink is in thermal contact with the heat absorber. The heat absorber collects the heat generated by the array of LEDs and provides a conductive path for the heat towards the heat sink for dissipation into the ambient environment. The disclosed heat absorber, however, is typically a poor heat conductor and does not provide for optimal heat transfer to the heat sink.
U.S. Pat. No. 5,173,839 to Metz, Jr. is directed to an LED array thermally bonded to a strip of alumina that is bonded to a heat sink bonded via thermally-conductive tape. Similarly, U.S. Pat. No. 5,857,767 to Hochstein teaches mounting LEDs on a metal core PCB having an integral heat sink with electrically and thermally conductive epoxy.
The optical performance of a light-emitting diode is another important consideration when designing high flux LED luminaires. The light-emitting diode used to generate light often has special emission characteristics. Optical devices such as reflectors or lenses have specific geometries which enable them to ameliorate the performance of the light-emitting diode. The performance of the LED can be improved by a judicious choice of optical devices adapted to particular output characteristics of the light-emitting diode.
Traditional directly viewed luminaires use light-emitting diodes with no optics and a housing comprising a transparent shield typically made of glass or plastic to protect the light-emitting diodes against natural elements. The transparent shield effectively blocks the light-emitting diode's output and reduces the overall illumination luminous flux output of the luminaire. Moreover, the individual light-emitting diodes are often visible through the transparent shield and could appear as point sources. This can further reduce light output uniformity and can cause a “pearl necklace” effect, which is undesirable.
A number of solutions have been proposed to alleviate the undesirable pearl necklace effect. One solution seeks to improve light output uniformity by providing a diffuse transparent shield surrounding the light-emitting diodes. However, in order to achieve good levels of luminous uniformity, the light-emitting diodes must be spaced relatively close with respect to one another. Due to design limitations, this solution is often not available, especially when using high flux light-emitting diodes whereby the close proximity of the light-emitting diodes creates a high concentration of unwanted heat. This problem is further exacerbated in luminaires having a plurality of light-emitting diodes of different colour combinations for colour mixing, where the distal spacing between the various light-emitting diodes must be minimized to generate a desired resultant colour.
Therefore there is a need for a new design for a directly viewable luminaire that can address these thermal and optical deficiencies identified in the prior art.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.